HISTORY OF MICROBIOLOGY
The Theory of Spontaneous Generation
The Greek philosopher Aristotle (384–322 BC) was one of the earliest recorded scholars to articulate the theory of spontaneous generation, the notion that life can arise from nonliving matter. Aristotle proposed that life arose from nonliving material if the material contained pneuma (“vital heat”). As evidence, he noted several instances of the appearance of animals from environments previously devoid of such animals, such as the seemingly sudden appearance of fish in a new puddle of water.
This theory persisted into the seventeenth century, when scientists undertook additional experimentation to support or disprove it. By this time, the proponents of the theory cited how frogs simply seem to appear along the muddy banks of the Nile River in Egypt during the annual flooding. Others observed that mice simply appeared among grain stored in barns with thatched roofs. When the roof leaked and the grain molded, mice appeared. Jan Baptista van Helmont, a seventeenth century Flemish scientist, proposed that mice could arise from rags and wheat kernels left in an open container for 3 weeks. In reality, such habitats provided ideal food sources and shelter for mouse populations to flourish.
However, one of van Helmont’s contemporaries, Italian physician Francesco Redi (1626–1697), performed an experiment in 1668 that was one of the first to refute the idea that maggots (the larvae of flies) spontaneously generate on meat left out in the open air. He predicted that preventing flies from having direct contact with the meat would also prevent the appearance of maggots. Redi left meat in each of six containers (Figure 1). Two were open to the air, two were covered with gauze, and two were tightly sealed. His hypothesis was supported when maggots developed in the uncovered jars, but no maggots appeared in either the gauze-covered or the tightly sealed jars. He concluded that maggots could only form when flies were allowed to lay eggs in the meat, and that the maggots were the offspring of flies, not the product of spontaneous generation.
Figure 1. Francesco Redi’s experimental setup consisted of an open container, a container sealed with a cork top, and a container covered in mesh that let in air but not flies. Maggots only appeared on the meat in the open container. However, maggots were also found on the gauze of the gauze-covered container.
In 1745, John Needham (1713–1781) published a report of his own experiments, in which he briefly boiled broth infused with plant or animal matter, hoping to kill all preexisting microbes.[2] He then sealed the flasks. After a few days, Needham observed that the broth had become cloudy and a single drop contained numerous microscopic creatures. He argued that the new microbes must have arisen spontaneously. In reality, however, he likely did not boil the broth enough to kill all preexisting microbes.
Lazzaro Spallanzani (1729–1799) did not agree with Needham’s conclusions, however, and performed hundreds of carefully executed experiments using heated broth.[3] As in Needham’s experiment, broth in sealed jars and unsealed jars was infused with plant and animal matter. Spallanzani’s results contradicted the findings of Needham: Heated but sealed flasks remained clear, without any signs of spontaneous growth, unless the flasks were subsequently opened to the air. This suggested that microbes were introduced into these flasks from the air. In response to Spallanzani’s findings, Needham argued that life originates from a “life force” that was destroyed during Spallanzani’s extended boiling. Any subsequent sealing of the flasks then prevented new life force from entering and causing spontaneous generation.
Finally it was Louis Pasteur who disproved spontaneous generation theory with his famous Swan — Neck Flask experiment.
Note: Swan — Neck Flask experiment is discussed in detail under Pasteur’s contributions to microbiology
CONTIBUTIONS OF IMPORTANT SCIENTISTS TO MICROBIOLOGY
Robert Hooke
Robert Hooke was born on the Isle of Wight, England on July 28, 1635 Active in the 17th century, Robert Hooke is one of the most important scientists of his generation and contributed in an amazing variety of scientific fields. Among other things, he was the first to discover the cell; Robert Hooke used an improved compound microscope he had built to study the bark of a cork tree. In doing so he discovered and named the cell – the building block of life. However he didn’t know its true biological function. Hooke coined the term cell and published the discovery in his famous 1665 book Micrographia. Hooke’s Micrographia, the first scientific best seller and one of the most important books ever written, demonstrated the tremendous power of the microscope and inspired people to use it for scientific exploration. Robert Hooke was the first person to use a microscope to study fossils and he published his findings in Micrographia. He concluded that fossils had once been living creatures whose cells had become mineralized. He also concluded that some species that had once existed must have become extinct. This was a controversial suggestion as most people at the time found the concept of extinction theologically unacceptable. Thus Hooke was one of the first proponents of a theory of evolution.
A drawing of Robert Hookes microscope
https://learnodo-newtonic.com/robert-hooke-contribution
Antonie van Leeuwenhoek (1632 — 1723)
Antonie van Leeuwenhoek was born on October 24, 1632, in the small city of Delft in the Dutch Republic . A moderately educated owner of a textile business, he learned how to make his own unique microscopes which offered unparalleled magnification. Using these microscopes he made a number of crucially important scientific discoveries, including single-celled animals and plants, bacteria, and spermatozoa. His microscopy methods were so finely tuned that after he discovered bacteria, this type of organism would not be observed again by any other scientist for over 100 years.
People in the textiles trade had, for hundreds of years, used glass pearls — small spheres of glass — as lenses to examine cloth in fine detail. Leeuwenhoek used glass pearls frequently in his day-to-day business to examine the density of threads and the quality of cloth.
In 1665 the great English scientist Robert Hooke released Micrographia, showcasing drawings he had made of the natural world seen through the lens of his microscope. Micrographia contains a description of how a powerful microscope could be made using a single spherical lens — similar to the glass pearls Leeuwenhoek was already familiar with. It is believed that Leeuwenhooke was inspired by Robert Hooke’s book and began making lenses for observing microscopic life.
Microscopes made from Leeuwenhoek’s tiny spherical lenses — the smallest lenses measured just 1 mm across — were easily capable of magnifying objects by a factor of about 200 — 300, while Hooke’s compound microscope magnified only by a factor of about 40 — 50.
Remarkably, Leeuwenhoek could use his lenses to resolve details as small as 1.35 μm. (This meant that, for example, he could easily see red blood cells, which are typically 6 — 8 μm in diameter.) Leeuwenhoek was a tradesman who had no formal training in science and had never been to college.Nevertheless, the quality of his observations was so high and his discoveries so compelling that his research became well-known through letters he sent to the Royal Society in London. These were translated into English and published in the Society’s journal, Philosophical Transactions.
In 1674, aged 41, Leeuwenhoek made the first of his great discoveries: single-celled life forms. He observed and recorded Red Blood cells, bacteria in water & soil, sperm cells & egg cells, lymphatic capillaries & life cycle of maggots. Antonie van Leeuwenhoek died aged 90 on August 26, 1723.
LOUIS PASTEUR ( 1822-1895)
Louis Pasteur (December 27, 1822 — September 28, 1895) was a French chemist and microbiologist noted for many of his significant findings in microbiology.
Some of the most important contributions made by Pasteur may be enlisted as follows,
Established that microorganism were responsible for fermentation as well as spoilage of wine
Developed the technique of Pasteurization for preservation of milk and other liquids.
Developed vaccines for fowl cholera, rabies
Coined the term ‘vaccine’ to commemorate Edward Jenner’s first developed vaccine against small pox
Disproved spontaneous generation theory by his famous ‘swan neck flask experiment’.
Contributed towards the understanding of causative agents and management of silkworm diseases
Pasteur started his career as a chemist in wine industry where he studied optical isomerism in tartaric acid crystals. It was here that he first established that microorganisms were responsible for the process of fermentation as well as spoilage of wine. He observed that wine could get ‘sick’ & become sour. He found that undesirable microbial growth was the reason behind ‘sick’ wine. Pasteur then developed the technique of pasteurization (patented in 1865) to fight the “diseases” of wine. He realized that these were caused by unwanted microorganisms that could be destroyed by heating wine to a temperature between 60° and 100°C. The process was later extended to all sorts of other spoilable substances, such as milk.
The observation that wine could get sick from bacteria led him to hypothesize that people’s sickness could also be attributed to bacteria. This and the conclusions made by many other contemporary scientists, led Pasteur to propose the ‘Germ Theory Of Disease’. It states that infectious diseases were caused by microorganisms.
He proved that microbes were attacking healthy silkworm eggs, causing an unknown disease, and that the disease would be eliminated if the microbes were eliminated. He eventually developed a method to prevent their contamination and it was soon used by silk producers throughout the world.
FOWL CHOLERA: In 1870’s Pasteur engaged in the studies of disease causing micro organisms . In his experiments on fowl cholera he inoculated the birds with the laboratory cultures and found that no birds could survive cholera.
Months into the experiments, Pasteur let cultures of fowl cholera stand idle while he went on vacation. When he returned and the same procedure was attempted, the chickens did not become diseased as before. Pasteur could easily have deduced that the culture was dead and could not be revived, but instead he was inspired to inoculate the experimental chickens with a virulent culture. Amazingly, the chickens survived and did not become diseased; they were protected by a microbe attenuated over time. Realizing he had discovered a technique that could be extended to other diseases, Pasteur returned to his study of anthrax. Pasteur produced vaccines from weakened anthrax bacilli that could indeed protect sheep and other animals.
RABIES VACCINE : Rabies had presented a new obstacle for Pasteur in the development of a successful vaccine. Unlike chicken cholera and anthrax, both caused by bacterium, the microorganism causing the disease could not be specifically identified, meaning Pasteur would not be able to develop the vaccine in vitro (in the laboratory).
Pasteur did not know this at the time, but the reason he could not find the microorganism is because rabies is a viral disease. Viruses are small infectious agents that replicate quickly and have a high mutation rate. These rapid mutations can be used to the benefit of researchers in the development of an attenuated vaccine. By serial passage of a virus through a different species, the virus becomes more adapted to that species, and less adapted to its original host, deceasing virulence with respect to the original host (e.g. it is “attenuated”). By passing the virus through rabbits, Pasteur made the virus less dangerous to human hosts, while still giving the body enough information to recognize the antigen and develop immunity to the “wild” version of the disease.
After successfully protecting dogs from the disease, Pasteur agreed to treat his first human patient, a nine-year-old boy who had been so severely attacked by feral dogs there was little doubt he would die if nothing was done. Pasteur injected the boy with a daily series of progressively more virulent doses of the vaccine from the rabies-infected rabbits. The boy never developed symptoms and Pasteur became an international hero.
Until Louis Pasteur developed the rabies vaccine, “vaccines” had referred only to the cowpox inoculation for smallpox. His procedure was originally called “Pasteur’s treatment”, but Pasteur decided to honor the 18th century virology pioneer Edward Jenner, who publicized the cure for smallpox, and give these artificially weakened diseases the generic name of “vaccines”. Thus, we largely have Pasteur to thank for today’s definition of a vaccine as a “suspension of live (usually attenuated) or inactivated microorganisms (e.g., bacteria or viruses) or fractions thereof administered to induce immunity and prevent infectious disease or its sequelae.”
Swan Neck flask experiment: In his famous experiment, Louis Pasteur used a special flask whose neck was shaped like an S or the neck of a swan, hence the name “Swan Neck Flask.” He put a nutrient rich broth in the flask, which he called the “infusion.” He then boiled the infusion killing any microorganisms which were already present. Then he allowed the infusion to sit. Because of the shape of the flask, the infusion was exposed to air. However, dust particles and other things in the air never made it into the infusion. Because they were trapped in the curve of the Swan Neck Flask. No matter how long he allowed the flask to sit, microorganisms never appeared in the infusion. However, if he tipped the flask and allowed the things trapped in the neck to get into the infusion then microorgranisms began to appear in the infusion and multiply rapidly. This demonstrates that microorganisms do not appear as a result of Spontaneous Generation. Instead, they are introduced into food through dust particles and other things that happen to land on the food.
ROBERT KOCH
Robert Koch was a German physician who is widely credited as one of the founders of bacteriology and microbiology. He investigated the anthrax disease cycle in 1876, and studied the bacteria that causes tuberculosis in 1882, and cholera in 1883. He also formulated Koch’s postulates. Koch won the 1905 Nobel Prize in Physiology or Medicine.
Major contributions of Koch may be listed out as following:
Identification of causative organism of anthrax as the bacterium Bacillus anthraces
Developed a technique of isolating bacteria as pure culture on solid media
An outstanding contribution to microbiology was proposal of Koch’s postulates
Development of tuberculin and studies on Tuberculosis
Contributions to the study of cholera
Koch also found a way to grow bacteria in pure cultures—cultures that contained only one kind of organism. He tried streaking bacterial suspensions on potato slices and then on solidified gelatin. But gelatin melts at incubator (body) temperature; even at room temperature. Finally, Angelina Hesse the American wife of one of Koch’s colleagues, suggested that Koch add agar (a thickener used in cooking) to his bacteriological media. This created a firm surface over which microorganisms could be spread very thinly—so thinly that some individual organisms were separated from all others. Each individual organism then multiplied to make a colony of thousands of descendants. Koch’s technique of preparing pure cultures is still used today.
Koch’s outstanding achievement was the formulation of four postulates to associate a particular organism with a specific disease. Koch’s Postulates are as follows,
The specific causative agent must be found in every case of the disease.
The disease organism must be isolated in pure culture.
Inoculation of a sample of the culture into a healthy, susceptible animal must produce the same disease.
The disease organism must be recovered from the inoculated animal.
Implied in Koch’s postulates is his one organism–one disease concept. The postulates assume that an infectious disease is caused by a single organism, and they are directed toward establishing that fact. This concept also was an important advance in the development of the germ theory of disease.
He also developed tuberculin, which he hoped would be a vaccine against tuberculosis. Because he underestimated the difficulty of killing the organism that causes tuberculosis, the use of tuberculin resulted in several deaths from that disease. Although tuberculin was unacceptable as a vaccine, its development laid the groundwork for a skin test to diagnose tuberculosis. After the vaccine disaster, Koch left Germany. He made several visits to Africa, at least two visits to Asia, and one to the United States. In the remaining 15 years of his life, his accomplishments were many and varied. He conducted research on malaria, typhoid fever, sleeping sickness, and several other diseases. His studies of tuberculosis won him the Nobel Prize for Physiology and Medicine in 1905, and his work in Africa and Asia won him great respect on those continents.
EDWARD JENNER
Also known as the “Father of Immunology”, Edward Anthony Jenner was an English scientist and is famous for his discovery of the smallpox vaccine. This was the first successful vaccine ever to be developed and remains the only effective preventive treatment for the fatal smallpox disease. His discovery was an enormous medical breakthrough and has saved countless lives. In 1980, the World Health Organization declared smallpox an eliminated disease.
Smallpox was unknown in Europe until the Crusaders carried it back from the Near East in the twelfth century. By the seventeenth century, it was widespread. In 1717 Lady Mary Ashley Montagu, wife of the British ambassador to Turkey, introduced a kind of immunization to England. A thread was soaked in fluid from a smallpox vesicle (blister) and drawn through a small incision in the arm. This technique, called variolation, was used at first by only a few prominent people, but eventually it became widespread. In the late eighteenth century, Edward Jenner realized that milkmaids who got cowpox did not get smallpox, and he inoculated his own son with fluid from a cowpox blister. He later similarly inoculated an 8-year-old and subsequently inoculated the same child with smallpox. The child remained healthy. The word vaccinia (vacca, the Latin name for “cow”) gave rise both to the name of the virus that causes cowpox and to the word vaccine.
Sergei Winogradsky
Sergei Nikolaievich Winogradsky was born on September 1, 1856 in Kiev, which was then in the Russian Empire . Sergei Winogradsky founded microbial ecology and he was a founding father of microbiology. The contributions of Winogradsky may be enlisted as follows,
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Discovered chemosynthesis – an entirely new mode of life, in which the energy to build organic molecules comes from chemical reactions rather than from sunlight in the more familiar photosynthesis.
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Invented the Winogradsky column.
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Discovered and isolated nitrogen fixing bacteria in soil that make nitrates available to green plants.
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Founded microbial ecology, where the interactions of microbes in cycles with their natural environments are studied holistically.
Winogradsky’s study on Beggiatoa, a puzzling bacterial organism led to ground breaking discovery of chemosynthesisin organisms. Beggiatoa was puzzling because it did not grow in the nutrient mixtures normally used in laboratories, yet in sulfurous spring waters it could develop into huge colonies, forming large mats.By 1887, the 31 year old Winogradsky had discovered that Beggiatoa did not obtain energy by any previously known method. Beggiatoa’s energy came from a chemical reaction between hydrogen sulfide, present in sulfurous waters, and oxygen. The reaction also produced sulfur and water.Beggiatoa stored the sulfur it produced within its own cells and, if needed, could react this sulfur with oxygen to produce even more energy.It was the first time anyone had discovered a living organism that survived using inorganic compounds/minerals as a source of energy. This type of organism is now called a lithotroph.
Winogradsky column
During his time in Strasbourg, Winogradsky invented the Winogradsky Column, which is still used today to learn which communities of bacteria are present in a sample. It consists of a glass cylinder half-filled with mud from a river or lake, topped up with water. A number of ingredients, such as sodium sulfate, calcium carbonate, and cellulose (newspaper) are added to provide sources of nutrition for bacteria.Within the column, scientists can control the amount of nutrients and light available to bacteria.
Construction of a winogradsky column:
The columns are easy to set up with a glass or perspex tube, about 30 cm tall and 5 cm diameter. Mud from the bottom of a lake or river is supplemented with cellulose (e.g. newspaper), sodium sulphate and calcium carbonate, then added to the lower one-third of the tube. The rest of the tube is filled with water from the lake or river, and the tube is capped and placed near a window with supplementary strip lights.
All the organisms are present initially in low numbers, but when the tubes are incubated for 2 to 3 months the different types of microorganism proliferate and occupy distinct zones where the environmental conditions favour their specific activities. The large amount of cellulose added initially promotes rapid microbial growth which soon depletes the oxygen in the sediment and in the water column. Different layers of the water column generates different zones of organisms that occupy that particular niche in the column.
Winogradsky enjoyed an unusually long and active life. He was 93 years old when he completed a 900 page book: Soil Microbiology: Problems and Methods, Fifty Years of Investigations. Sergei Winogradsky died in his sleep at the age of 96 in Brie-Comte-Robert, France, on February 25, 1953.
USEFUL WEBLINKS & REFERENCES
Jacquelyn Black′s 8th Edition of Microbiology: Principles and Explorations
Prescott’s Microbiology
Brock Biology of Microorganisms (14th Edition)
Ananthanarayan and Paniker’s Textbook of Microbiology
September 25, 2020
CELL MIGRATION
Cell migration is a broad term that we use to refer to those processes that involve the translation of cells from one location to another. This may occur in non-live environments, such as soil or within complex, multicellular organisms. Cells migrate in response to multiple situations they encounter during their lives. Some examples include: the need to feed ; morphogenetic events that require the mobilization of precursors to generate new structures/layers/organs, sometimes at distant locations (during embryogenesis, organogenesis and regeneration); or the presence of environment cues that inform the cells of the need for their movement to accomplish a larger goal (e.g. wound healing or the immune response). In pathology, production of abnormal migratory signals may induce the migration of the wrong cell type to the wrong place, which may have catastrophic effects on tissue homeostasis and overall health.
Some examples include autoimmune syndromes in which immune cells home to certain locations (joints in rheumatoid arthritis, and the CNS in multiple sclerosis are two examples) and destroy the supporting tissue, causing severe damage; or the process of metastasis, in which tumor cells abandon the primary tumor and migrate to distant tissues where they generate secondary tumors.
There are different modes of cell migration depending on the cell type and the context in which it is migrating. Cells can move as single entities, and the specifics of their motility depend on several factors, e.g., adhesion strength and the type of substratum (including extracellular matrix ligands and other cells), external migratory signals and cues, mechanical pliability, dimensionality, and the organization of the cellular cytoskeleton. The intrinsic properties of the cell interact with the environment to produce a migratory mode or phenotype. For example, nimble, fast-moving and -turning cells, like immune cells, do not have a highly organized cytoskeleton and tend to adhere weakly; their motion is sometimes termed ‘amoeboid’. Some tumor cells can move by extending membrane blebs, and their actin cytoskeleton is not very organized, either. Fibroblasts and epithelial precursors lie at another extreme. They have elaborate cytoskeletal structures and adhesions, and their motion is generally slow. It is worth noting that some cell types can switch between these depending on their environment. Cells can also move in groups, including chains of cells and sheet-like layers.
Deconstructing Cell Migration: Overview of its Component Processes
Polarization
Cell polarization refers to the tendency of migrating cells to have a distinct, stable front and rear. The polarity is reinforced and often even arises from environments that provide a directional cue. These directional cues can be chemotactic, (induced by chemoattractants or morphogens), mechanotactic (breakdown of cell-cell contacts, as in wound healing), electrotactic (induced by electric fields) or a combination of any of these.
The leading edge is usually characterized by intense actin polymerization that generates a protrusive structure, and by adhesion to the substratum. The trailing edge is characterized by stable bundles and the release and disassembly of adhesions. The central part of the cell usually contains the nucleus and microtubules (which exhibit different degrees of polarization depending on the cell type).
Protrusion
Protrusion is the de novo formation of membrane extensions, or protrusions, in the direction of migration, i.e. the leading edge. It has three major components: the expansion of the plasma membrane, the formation of an underlying backbone that supports membrane extension, and the establishment of contacts with the substratum, which provides traction for the movement of the rest of the cell body and signals that regulate actin polymerization.
The protrusion is produced by local actin polymerization. One kind of protrusion is flat and fan-like, the edge of which is often called the lamellipodium and within which actin is polymerizing and often branched. Spike-like filopodia are another kind of protrusion; these structures comprise polymerized actin filaments that are arranged into long parallel bundles. These two forms of protrusion are thought to serve different roles: filopodia act as mechanosensory, exploratory devices, whereas lamellipodia provide wide surfaces that generate traction for forward movement.
Adhesion
Adhesion to the substratum occurs mainly via integrin receptors. The integrins are a large superfamily of heterodimeric receptors that bind to different extracellular matrix ligands or counter receptors on other cells. Integrin ligation triggers signaling pathways that regulate protrusion. It also links the substratum to the actin cytoskeleton and thereby provides traction for migration. The sites of adhesion are usually spatially restricted and vary from small and dot-like (nascent adhesions or focal complexes) to large and elongated (focal adhesions). The shape, size and functional role of the adhesions vary with their subcellular localization and cell type. Those closer to the leading edge, i.e. embedded in the lamellipodium, or present in rapidly migrating amoeboid cells tend to be smaller, actively promote actin polymerization and assemble and disassemble rapidly. Those further away from the leading edge in mesenchymal cells can be larger, more stable and anchor large actin filament bundles.
Over 150 different molecules populate adhesions. Some are organized into signaling complexes that contain kinases and adapter proteins that serve to bring different signaling components together. Paxillin and FAK are two among many important signaling components in adhesions. Another group of adhesion components links actin to the substratum through integrin. They include talin, vinculin, and α-actinin.
Migration in Health and Disease
Cell migration is fundamental to the morphogenesis of embryos. Migratory movements underlie gastrulation, the formation of the layers in the embryo, as well as the formation of organs and tissues. In addition to this morphogenetic component, cell migration is a key component of the homeostasis of the adult individual. Two common themes are the migration of cell sheets and the birth of undifferentiated cells in epithelial layers, and their migration to distant targets. The former is a prominent feature of gastrulation. Examples of the latter are migrations from the neural tube (and neural crest) and the somites. This provide cells that populate numerous organs and tissues including skin, brain, and limbs. Tissue regeneration and repair is a prominent homeostatic phenomenon in skin and intestine, for example. And the inflammatory cascade, which fights off disease throughout the body, involves the movement of immune cells from the lymph nodes to the circulation where they remain vigilant until tissue insult triggers an inflammatory reaction that attracts them to respond to insult, i.e., injury or infection.
Consequently, failure of cells to migrate, or inappropriate migratory movements, can result in severe defects (during development) or life-threatening scenarios, such as immunosuppresion, autoimmune diseases, defective wound repair, or tumor dissemination. Understanding the mechanisms underlying cell migration is also important to emerging areas of biotechnology which focus on cellular transplantation and the manufacture of artificial tissues, as well as for the development of new therapeutic strategies for controlling invasive tumor cells.
September 24, 2020
Human Evolution
Modern humans are a young species in geological terms. The earliest fossils that meet the criteria for archaic Homo sapiens, the genus and species name for humans today, date to about 400,000 years ago, while modern humans have been around for perhaps 170,000 years or so.
The apes that would ultimately evolve into the humans of today split from the so-called lesser apes about 7 million years ago. These are the Hominidae, or great apes. This is the approximate time frame given for the divergence of the human lineage from that of chimpanzees, humans’ closest surviving relatives. This divergence is believed to have taken place in Africa, with many early hominid fossils gathered in Kenya. Several different candidates exist in terms of which organism ultimately evolved into modern humans rather than ultimately dying off.
It was in the Miocene age that the family Hominidae split from the Pongidae(apes) family. Dryopethicus was the first in the evolution of man in the stages of evolution and some believe him to be the common ancestor of man and apes.
Dryopethicus:
He was the earliest known ancestor of man. Dryopethicus inhabited the European region and some parts of Asia and Africa. Dryopithecus is one of 40 genera representing up to 100 species of extinct apes that lived during the Miocene (22.5 to 5.5 million years ago). Dryopithecus fontani was the first fossil great ape discovered. It was discovered in France in 1856. Like all living apes, dryopithecines possessed relatively large brains. They also show apelike characteristics associated with a reduced reliance on smell and an increased emphasis on vision: they had shortened snouts and forward-facing eye sockets with overlapping fields of vision. Like all living apes, dryopithecines also lacked a tail. The skeletal remains indicate that dryopithecines were quadrupeds, walking on four legs. Scientists suggest that Dryopithecus was the likely ancestor of African apes and humans.
Australopithecus Afarensis:
This species lived between 3.9 and 2.8 million years ago. This species is one of the longest lived and well known. They are known to have survived for 900,000 years. This ancestor of modern humans would have been recognized as having both ape-like and human-like features. The earliest example was discovered in southern Africa in 1924 . Australopithecus means ‘southern ape’ and was originally developed for a species found in South Africa.
The word afarensis is based on the location where some of the first fossils for this species were discovered — the Afar Depression in Ethiopia, Africa ‘Lucy’ the famous fossil belonged to this species.
Australopithecus was 1.2 meters tall ( less than 4 feet ) . The fossils show the foramen magnum that was large to indicate upright walking. But still it retained many ape-like features including adaptations for tree climbing, a small brain, and a long jaw. The brain was small, averaging approximately 430 cubic centimetres .The forelimbs were different from those of the earlier ape-like ancestors. They had teeth like humans.
Homo Habilis — ’The handy man’:
Homo habilis, known as ‘handy man’ is a species of the genus Homo which lived from approximately 2.33 to 1.4 million years ago, during the Pleistocene period. The discovery and description of this species is credited to both Mary and Louis Leakey, who discovered the fossils in Tanzania between 1962 and 1964. This species is believed to have been ancestors of Homo erectus. Homo habilis is known as the ‘handy man’ because he was the first to make and use tools. The word habilis is based on a Latin word meaning ‘handy’ or ‘skilful’ . The first crude stone tools consisting of simple choppers, core tools and scrapers were made as early as 2.6 million years ago and are classified as Mode 1 technology.
Homo habilis had a larger brain than earlier human ancestors and this is reflected in significant changes to the shape of the skull. Brain averaged 610 cubic centimetres in size, representing 1.7 per cent of their body weight. Foramen magnum for the spinal cord was located in the centre of the skull base, showing that this species walked on two legs. Homo habilis ranged from about 3 1/2 to 4 1/2 feet tall, but weighed only about 70 pounds.
Homo erectus:
Fossils of these short and stocky humans, with their distinctive skull shape and large brow ridges, have mostly been found in China and Indonesia. This species lived between 100,000 and 1.6 million years ago, although some estimates extend this to between 35,000 and 1.8 million years ago. After years of searching Indonesia for ‘the missing link’, Dutchman Eugene Dubois finally uncovered part of a skull in 1891 (known as ‘Java Man’). He believed this fossil belonged to an ancient and ‘upright’ human and so coined the species name erectus.
Homo erectus is now one of the better known of our human relatives with over 40 specimens excavated from Java and many more from sites in China.Their brain showed an increase in size over earlier species and averaged about 1050 cubic centimetres. The structure of the brain was similar to that of modern humans. The face was large with a low, sloping forehead, a massive brow ridge and a broad, flat nose. limbs were like those of modern humans although the bones were thicker, suggesting a physically demanding lifestyle.
The remains of meals have been found at some Homo erectus sites in China. These show that they ate large amounts of meat supplemented with plant foods and, in general, had a diet similar to that of early modern humans. Moreover, there is evidence [bone protein released by hominid fire] to suggest the use of fire to cook food. This would produce a higher-energy diet, reallocating calories, and encouraging brain growth.
HOMO NEANDERTHALENSIS – THE NEANDERTHALS:
Neanderthals co-existed with modern humans for long periods of time before eventually becoming extinct about 28,000 years ago. This species lived between 28,000 and 300,000 years ago. A 45,000-year-old skullcap discovered in 1856 in Feldhofer Grotto, Neander Valley, Germany. This is the ‘type specimen’ or official representative of this species. Remains of this species have been found scattered across Europe and the Middle East. The eastern-most occurrence of a Neanderthal may be represented by a fossil skull from China known as ‘Maba’.
Neanderthals had more robust skeletons and muscular bodies than modern humans.Males averaged about 168 centimetres in height while females were slightly shorter at 156 centimetres.Back of the skull had a bulge called the occipital bun and a depression (the suprainiac fossa) for the attachment of strong neck muscles. jaws were larger and more robust than those of modern humans . Teeth were larger than those of modern humans.Limb bones were thick and had large joints which indicates they had strongly muscled arms and legs.
Evidence shows that Neanderthals had a complex culture. The Neanderthals had a reasonably advanced tool kit classified as Mode 3 technology that was also used by early members of our own species, Homo sapiens. The Neanderthals built hearths and were able to control fire for warmth, cooking and protection. They were known to wear animal hides, especially in cooler areas. However, there is no physical evidence that Neanderthal clothing was sewed together, and it may have simply been wrapped around the body and tied. Caves were often used as shelters but open air shelters were also constructed. The dead were often buried, although there is no conclusive evidence for any ritualistic behaviour. Neanderthals were very good hunters too.
Homo sapiens — The Modern Humans:
Homo sapiens represents us, the modern humans. Our species has existed for the past 160,000 years. ‘Cro-Magnon Man’ is commonly used for the modern humans that inhabited Europe from about 40,000 to 10,000 years ago. The name we selected for ourselves means ‘wise human’. Homo is the Latin word for ‘human’ or ‘man’ and sapiens is derived from a Latin word that means ‘wise’ or ‘astute’.
Fossils of the earliest members of our species, archaic Homo sapiens, have all been found in Africa. African fossils provide the best evidence for the evolutionary transition from Homo heidelbergensis to archaic Homo sapiens and then to early modern Homo sapiens.
Homo sapiens skulls have a distinctive shape that differentiates them from earlier human species. The earliest Homo sapiens had bodies with short, slender trunks and long limbs. Homo sapiens living today have an average brain size of about 1350 cubic centimetres which makes-up 2.2% of our body weight. Early Homo sapiens, however, had slightly larger brains at nearly 1500 cubic centimetres. Unlike other species of Homo, the skull is broadest at the top. The back of the skull is rounded and indicates a reduction in neck muscles. Face is reasonably small with a projecting nose bone. Jaws are short which result in an almost vertical face.Teeth are relatively small compared with earlier species.
September 24, 2020
BiologyClassroom
DNA ( Deoxyribonucleic acid) is a biomolecule that is found inside the nucleus of every cell ( there are a few exceptions ) of all living organisms. This molecule is made up of ribose sugar which is nothing but a 5 carbon sugar, phosphate group and nitrogenous bases. Now these nitrogenous bases are really important because they are the ones that code for proteins. In other words they are the building blocks of the DNA molecule. The sequence in which they get arranged is what gives meaning to the DNA. i.e. their sequence represents what is called the genetic code.